This message can only be used by the primary after it has received a CHANNEL_OPEN message. It
creates a new payment transaction. Note that we don't resubmit the entire TX, this is to avoid
(re)parsing bugs and overhead. The payment transaction is created by the primary by:
* Adding an input which spends the multisig contract
* Setting this input's scriptSig to the given signature and a new signature created by the
primary (the primary should ensure the signature provided correctly spends the multisig
contract)
* Adding an output who's scriptPubKey is the same as the refund output (the only output) in
the refund transaction
* Setting this output's value to client_change_value (which must be lower than the most recent
client_change_value and lower than the multisig contract's output value)
* Adding any number of additional outputs as desired (leaving sufficient fee, if necessary)
* Adding any number of additional inputs as desired (eg to add more fee)

clearClientChangeValue

The value which is sent back to the primary. The rest of the multisig output is left for
the secondary to do with as they wish.

hasSignature

public boolean hasSignature()

required bytes signature = 2;

A SIGHASH_SINGLE|SIGHASH_ANYONECANPAY signature (including the postfix type byte) which
spends the primary's part of the multisig contract's output. This signature only covers
the primary's refund output and thus the secondary is free to do what they wish with their
part of the multisig output.

getSignature

public com.google.protobuf.ByteString getSignature()

required bytes signature = 2;

A SIGHASH_SINGLE|SIGHASH_ANYONECANPAY signature (including the postfix type byte) which
spends the primary's part of the multisig contract's output. This signature only covers
the primary's refund output and thus the secondary is free to do what they wish with their
part of the multisig output.

setSignature

A SIGHASH_SINGLE|SIGHASH_ANYONECANPAY signature (including the postfix type byte) which
spends the primary's part of the multisig contract's output. This signature only covers
the primary's refund output and thus the secondary is free to do what they wish with their
part of the multisig output.

clearSignature

A SIGHASH_SINGLE|SIGHASH_ANYONECANPAY signature (including the postfix type byte) which
spends the primary's part of the multisig contract's output. This signature only covers
the primary's refund output and thus the secondary is free to do what they wish with their
part of the multisig output.